Oceanside is a seashore town situated on the South Coast of California. This is the third-largest town in California's San Diego County. The town has a population of over 167,086 It forms a tri-city area along with Carlsbad and Vista. Oceanside is just south of the Pendleton Marine Corps Base Camp. Traveling north on Interstate 5, Oceanside is past Orange County's last place. As the crow flies, the drive from Aliso Viejo is about the same as that to downtown San Diego.